Low tension surface discharge spark plug



March 24, 1959 w. B. SMITS LOW TENSION SURFACE DISCHARGE SPARK PLUG Filed Aug. '12, 1954 IIIIJL ATTORNEY United States PatentOffice 2,879,426 Patented Mar. 24, 1959 LOWTENSIION. SURFACE DISCHARGE SPARK'PLUG Wytze- B. vSmits, Voorburg, Netherlands, assignor'to Smitsvonk, ..N-.V.,'Leidschendam, Netherlands, a com- Pa s Application (August 12, 1954, Serial No. 449,358

4 Claims. (Cl. 313-130) This invention relates to a low tension surface discharge spark plug, and more particularly to the construction of the sparking element for such a plug.

The-low tension ignition system to which the invention ,relates is described in my Patent No. 2,506,472 and-uis'the type in which a condenser is periodically charged from a voltage source and then discharged across the-electrodes of a surface discharge spark plug with a hot intense spark.

A -known low tension surface discharge spark plug is described in my Patent No. 2,625,922 and consistsprimarily of two concentric circular electrodes, between which is disposed a separating body of insulating or semiconducting material. The hot spark from the condenser discharge burns across the surface of the separating body at the point of least resistance between the electrodes. This is -a fundamental feature of spark plugs-of this typeand one which must be given primary-consideration in the design of such spark plugs:

The second fundamental aspect of these spark plugs ;is that their life depends upon-a uniform burning away of the separating body. If the spark across the separating body-was always at one point, and the separating body was burned away at that, pointdeep in between the electrodes,

the life of the plug wouldbe extremely short. On theother'hand if-the spark occurred at varying locations around the :circle betweentheelectrodes, the separating body: would be burned uniformly and gradually away, and the life of the plug would be greatly extended; a

The construction shown in Patent No. 2,625,922 is designedx'to permit the uniform burning away of the separating body and includes particularly the feature of forming a small angle between the facingsurface of the electrodes; Theangle diverges inwardly from the sparking surface of the plug whereby, as the separating body is burned away ata point the resistance between the electrodes at that point is increased because of the increased length of the gap at that point. The spark will therefore tend tomove 'to another portion. of the circle of less re-- sistance. This movement of the spark continues throughout the life of the spark plug and thus the burning away of the separating body takes place uniformly.

While the arrangement shownin Patent No. 2,625,922 has proved highly satisfactory, it has the disadvantage that the electrodesmust be precisely positioned with respect to each other, since any eccentricity in alignment will-result in the sparking occurring only at one point (the point of least resistance). Since the precise alignment of the electrodes makes the manufacturingprocess rather laborious, it would obviously be advantageous to provide a sparking element constructed so that the burning of the separating body takes place uniformly without requiring unusual precision in manufacturing.

Accordingly, it is an object of my invention to provide a low tension surface discharge spark plug which is simple to construct of inexpensive materials and which has a long life.

To accomplish: thisobject I utilize the fact thatthe resistancebetween'electrodes can be decreased by dispersing metallicpartieles in the separating body. If the metallic particles are dispersed regularly in a predetermined manner, the uniform burning away of the separating body can be assured.

According to the invention the separating body is made from a thin elongated tape of mica or other insulating material. Parallel strips or parallel lines of dots of electrically conductive material are disposed on the surface. of the tape-by imprinting, evaporating, sprinkling or the like. The tape is thenrolledinto a tight cylinder and pressed in betweenthe electrodes at such a great pressure that the parallel strips are entirely pressed into the mica (or-may be-previously embedded in the mica) so that the thickness of the mica through the strips is narrower thanzthe'thicknessof the mica through the layers vbetween the. strips; The separating body will thus be composed of the insulating material and a number of layers, parallel tothe active surface, of conductive material.

In "operation, the spark will 'tend to burn away the separating body uniformly at the first layer until that layeris'; completely burned away, before moving to the second layer, since the resistance at the first layer is less than thetresistance of the separating body between layers. The burning into the separating body atone point cannottakeplace as the insulating layer between the electrode at the place where the metallic material is found is narrower than :insulating material below each layer. The spark Jtherefore ceases .burning in deeper as soon as it reaches .the insulatingmaterial below a layer of metallic materiaL.

Anotherfeatureof the invention is that one or both electrodes may be thin-walled. The thin-walled electrode or electrodes will burn off gradually with the separating I body sothat the separating body isaways clearly exposed to the combustion mixture. If both electrodes were thick-walled,-the sparking would gradually occur farther and farther downin between the electrodes until it reached a pointat which-the spark would not be exposed to the combustion mixture,- and the life of the element would be ended.

It isan ;.object of my invention to provide a surface discharge spark plug element having its electrodes separated; by an-hinsulating body in which conductive material is disposed in spaced layers parallel to the sparking surface.

It .is'anotherobject-of myinvention to provide a sur-- face disehatrgespark'plug inwhich the requirements for precisiom in assembling the sparking element can be sub-' stantially, reduced-.-

It tisxanoth'er. object of my invention to provide a surface:discharge.:-spark plugelement having its electrodes separated .by-aninsulating or semi-conductive body and in which at least one of the electrodes is thin enough to be burned away; with the separating-body during sparking.

These;androtherobjects will become more readily apparent from-,thefollowing description taken with the accompanying drawings in which:

Fig. 1 is a fragmentary cross-sectional view of a sparkingelement; according to: the invention in which the electrodeshave the-same thickness.

Fig. 1a is a fragmentary cross-sectional view of the- Figs. 6 and 7 show two embodiments of the separating body in developed position.

Figs. 1 and la show the only part of the sparking element to which the present invention is directed. The element consists of the inner electrode 3, the outer electrode 4, said electrodes having the same thickness in this embodiment. A separating body 6 is inserted in the cylindrical space between the electrodes 3 and 4. The separating body consists, for example, of a tape of mica rolled into a cylinder and upon which tape conductive dots or strips 7 are placed.

The resistance between the electrodes at the location of the dots or strips (through line A) is smaller than the resistance of the separating body between the layers of dots or strips (through line B) so that the sparks between the electrodes will tend to occur at the conductive layers until the whole layer is burned off. Thereafter, the insulation under the layer will burn off until the next series or layer of dots or strips is reached.

The spark plug shown in Fig. 2 consists of the usual type of spark plug body 1 which is provided with a screw thread 2 by which the body can be screwed into the engine block. The sparking element consists of a solid central electrode 3, an outer electrode 4, the protruding end of which has a very thin wall 5, and a separating body 6. The separating body 6 is formed with the electrically conductive strips or small dots 7 according to the invention.

The sparking element is mounted in the spark plug body by clamping it on to a porcelain insulator 8. The mounting of the sparking element to the spark plug body is efiectcd by a central spindle 9 which is screwed into the inner end of the sparking element at one end thereof and is provided at the other end thereof with the nut 10 and the terminal nut 11 for the connecting cable. The insulator 8 is clamped on to the spark plug body by a nut 13 with a joint ring 12 disposed between the insulator 8 and the nut 13. Joint rings 14 and 15 may be mounted between the insulator 8 and the sparking element and between the sparking element and the spark plug body respectively.

In Fig. 6 the separating body is shown in developed position and consists of a mica tape 25. On the mica tape strips 26 of electrically conductive material, for example, metal, are provided by imprinting, sprinkling, evaporating or the like.

Fig. 7 shows an alternative construction for the separating body which consists of a mica tape having very small electrically conductive dots 27 disposed thereon in spaced parallel lines as shown.

The mica tape of these embodiments is rolled up and forms a thin tube which is inserted between the electrodes 3 and 4 as shown in Fig. In for example. During the sparking, the mica will burn off uniformly, that is the mica will burn down to the first layer of conductive material at which time the burning will continue to take place until all of the conductive material is burned 05 and then the sparking will take place down to the second layer of conductive material and so forth.

When the sparking element is formed with at least one thin wall, such as the wall 5 shown in Fig. 2, the thin wall will be burned off along with the separating body, the burning taking place along circles indicated at 16, 16', 16", etc. Thus, because the thin wall 5 burns ofi, the spark will always be quite plainly exposed to the combustion mixture no matter how far down the sparking element the separating body is burned off.

If, on the other hand, the walls of the electrodes were quite thick, only the separating body would be burned off and after a period of time a circular pit would be formed in the electrode at the bottom of which the sparking would be occurring. At a certain depth of this pit, the spark would no longer reach the combustion mixture and the life of the plug would be ended.

In Fig. 3 a bottom view of the sparking element is shown in which one of the metallic strips is visible as indicated at 17. This view shows that if the sparking element lacks precision in its manufacture, it is possible that the resistance at two places will be equal to each other and smaller than at all other points on the circle, and the spark in this case will not be a singular one but will be divided into two parts 18 and 19. Sparking will continue at these locations until the metallic strip at that location is burned away. Then the spark will move to another location on the circle where the resistance is smallest.

In Fig. 4 the sparking element is shown separately. This sparking element may be provided with a gas chamber (Fig. 5) and in this case the outer electrode 21 has the thicker wall and the inner electrode 22 has a very thin wall 23. The sparks appear in the gas chamber 20 as shown at 24.

In a general manner while I have in the above description disclosed what I deem to be practical and eflicient embodiments of my invention, it should be well understood that I do not wish tobe limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

I claim:

1. A sparking element for a low tension surface discharge spark plug comprising, in combination, a pair of spaced electrodes, insulating material separating said electrodes, the outer surfaces of said electrodes and insulating material forming a sparking surface, a plurality of particles of electrically conductive material disposed in said insulating material, said particles being disposed in a plurality of layers, said layers being spaced from each other along the longitudinal axis of said element and parallel to the sparking surface of said element whereby the insulating material separating the electrodes comprises in depth alternate layers of increased and decreased conductivity.

2. A sparking element according to claim 1 in which said particles comprise strips of electrically conductive material.

3. A sparking element according to claim 1 in which said particles comprise small dots of electrically conductive material.

4. A sparking element according to claim 1 wherein the end of one of said electrodes is formed with a cavity providing a gas chamber, the axially extending wall of said electrode being very thin, and in which the other electrode has a thick wall.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,576 Gorny et al Sept. 6, 1938 2,168,206 Hastings Aug. 1, 1939 2,640,473 Smits June 2, 1953 FOREIGN PATENTS 509,563 Belgium Mar. 15, 1952 

